In prior art systems earth stations can transmit data signals to remote earth stations via a satellite transponder. Similarly, earth stations can receive transmissions from distant earth stations via a satellite. In these systems many earth stations access the same satellite transponder so that the transponder is being used in a multiple access mode. In the prior art, several multiple access techniques are available. The two techniques most commonly employed are time division multiple access (TDMA) and frequency division multiple access (FDMA). In a system using relatively small earth station antennas, (from 3-4 feet in diameter) both of the foregoing techniques present substantial difficulties. The basic problem with both TDMA and FDMA lies in their interference protection. Small earth stations of the intended size experience substantial interference on reception and on transmission both in terms of interfering with other services or being interfered with because the small antenna size implies low antenna selectivity, i.e., the bore site radiated power relative to the off-axis radiated power.
In a TDMA system the data to be transmitted is buffered and then sent in bursts during a relatively short time interval or window on a periodic basis. Consequently, the average power over the entire period may be low because most of the time no transmission occurs. But during the narrow time windows, when data is being transmitted, the power is relatively high.
In FDMA, the data is transmitted continuously, but since the radiated signal is confined to a small section in a frequency band (so that others can use adjacent bands) the power is concentrated in portions of the frequency spectrum rather than in time as is the case with TDMA transmission.
Hence, especially in terms of the interference potentials from the earth station into other terrestrial services or to adjacent satellites, the concentration of transmitter power in either the time or frequency domains is undesirable. On the other hand, spread spectrum transmission has a unique distinction in that the power is not concentrated in either time or frequency. Many users can use the same bandwidth simultaneously in the spread spectrum multiple access mode (SSMA). The power produced by a spread spectrum transmitter is relatively constant over time and is spread out over a large frequency range. Depending upon how large such frequency range is, the actual level of any given signal can be lower than the thermal noise received at a given receiver with which the spread spectrum transmission might interfere. Hence, spread spectrum is an extremely desirable modulation method for multiple access of small earth stations.
The spread spectrum transmission employed in the present invention is the so-called direct sequence method. In the direct sequence method, a bit of information (data bit) is transmitted as a phase shift keyed transmission of a carrier with the phase shift keying being at an extremely rapid rate compared to the data bit rate since there are many time elements per bit. These time elements are conventionally called chips. The signal transmitted in the direct sequence method technique has a unique shift register pattern associated with it, usually called a pseudo random sequence (PRS). The PRS signal is a sequence of high and low level signals defined by the chips each of which is of equal time length, arranged in a random fashion, and representing the phase shifting of the carrier. If a binary 1 is to be transmitted the uninverted PRS signal is employed to modulate the carrier. If a binary 0 is to be transmitted the inverted PRS signal modulates the carrier.
In a typical application of the direct sequence method employing a PRS signal there might be 500-1000 chips in the PRS pattern. The bandwidth occupied by the signal is directly determined by the chip rate which is, in effect, a pseudo data rate. A receiver receiving a PRS signal from a given transmitter has the same PRS pattern stored therein. This stored PRS pattern can be employed to decode and extract the transmitted data even when there are many other stations using the same frequency band at the same time because the other stations are all using PRS signals of different patterns.
While the spread spectrum technique is extremely desirable from the point of view of reducing interference probability from a transmitting station and also from the point of view of reducing interference potential on reception, the efficiency of multiple access spread spectrum as it is conventionally used, i.e., with each station being asynchronous with each other station, is quite low compared with either TDMA and FDMA. In both TDMA and FDMA the signals transmitted by the individual stations are orthogonal to one another, i.e., they either occur at different times or in different frequency bands.
In conventional spread spectrum the waveforms are not orthogonal to each other and a station receiving a desired spread spectrum transmission will also see many other spread spectrum transmissions. While the other spread spectrum transmissions will appear as noise such noise forms a background which makes error probability high unless the number of simultaneous users in the band is kept reasonably low. The primary reason for the above-mentioned difficulties in the prior art spread spectrum systems results from the fact that the transmissions are not synchronized with respect to a common reference.
It is a primary purpose of the present invention to synchronize all of the chips and all of the bits of all transmission sequences of the various transmitters, which will result in the orthogonality of all such sequences when the sequences are properly designed. This will provide a multiple access efficiency of spread spectrum as high as for TDMA and FDMA systems. A further advantage of the present invention is that the interference protection is greater than that obtainable with the more conventional TDMA and FDMA systems and similar to that of ordinary, unsynchronized spread spectrum systems.